When drilling a well, the operator attaches a drill bit to the lower end of a string of drill pipe and rotates the drill bit, typically by rotating the drill string. The operator pumps drilling fluid down the drill pipe, which exits nozzles of the drill bit. The drilling fluid, along with cuttings, flows back up the annular space surrounding the string. The operator filters the cuttings from the drilling fluid and pumps the cleansed drilling fluid back down the drill pipe in continuous circulation.
The drilling fluid in most wells is weighted with a density that provides a hydrostatic pressure greater than the expected pressure of the earth formation being drilled. Making the drilling fluid hydrostatic pressure greater than the formation pressure reduces the chance of a blowout. In a blowout, the formation pressure exceeds the hydrostatic pressure of the drilling fluid and pushes the drilling fluid out of the hole, sometimes even with the drill pipe.
In some wells, the use of heavy drilling fluids causes excessive amounts of the drilling fluid to enter into the formation. Not only is the drilling fluid lost, but damage to the formation can occur. In another technique, called “underbalanced drilling”, the drilling fluid density is light enough so that the hydrostatic pressure at any point along the open hole portion of the well is less than the formation pressure. A rotating blowout preventer seals the upper end of the drill pipe to prevent a blowout. The rotating blowout preventer provides a seal even when the drill pipe is rotating. Underbalanced drilling avoids damage to the formation due to heavy drilling fluid.
To applicants' knowledge, underbalanced drilling has not been utilized in offshore drilling operations. In a typical offshore drilling operation, the operator will extend a drilling riser assembly from a wellhead housing at the sea floor to the drilling platform. The drilling riser assembly includes a subsea blowout preventer that connects to the wellhead housing. During conventional drilling, the drill string is lowered through the riser into the well. The drilling fluid is pumped from the drill pipe and returns up the drilling riser to a diverter at the drilling platform. The diverter diverts the circulating drilling fluid over to the filter equipment for removing cuttings. The diverter also has a blowout preventer that may be operated when the drill pipe is stationary in the event of an emergency.
The drilling riser is a large diameter string of pipe made up of sections that are secured together, typically by flanged connections. A conventional drilling riser possibly may not have a pressure rating adequate to withstand the higher pressure that would occur if the drilling fluid were significantly underbalanced.